Power tools, such as hand-held electric drills, are well known. Many modern electric drills now have a drill speed control mechanism whereby the drill can be rotated at two different speeds. Obviously, the lower speed is generally used for drilling larger holes whereas the higher speed is used for drilling holes of smaller diameter.
Further, many such drills are provided with a hammer mode so that they can be used either in a drill mode or in a hammer drill mode. In such drills the main shaft of the drill is displaceable in a longitudinal direction, usually against the force of a spring. Such displacement is often effected by rotating a control member which cooperates with a pin member, the pin member carrying a longitudinally extending, eccentric projection portion at its end remote from the control member. At the front end of the main shaft, two sets of ratchet teeth are provided. When the drill is in its "drill-only" mode, these sets of ratchet teeth are spaced apart and do not co-operate with one another. When, however, the drill is in its "hammer-mode" the ratchet teeth can engage with one another such that one of the sets of ratchet teeth associated with the chuck of the drill so as to rotate therewith rides over the crests and troughs of the other set of ratchet teeth which is usually formed on a fixed collar member surrounding the shaft. It will therefore be apparent that, although the drill still continues to rotate, the chuck, and therefore the drill bit, will also be moved in a reciprocating manner to provide a hammering effect.
The selection of the speed of rotation and/or the mode of operation is, as far as the user is concerned a matter of the utmost simplicity. Almost invariably, the or each selector is in the form of a control member, such as a button, located on the exterior of the housing of the drill. The user merely rotates the control member into an identifiable position.
It will, of course, be appreciated that it is of importance that once the drill speed and/or mode of operation has been selected, these parameters should not be variable other than at the will of the user. It is, therefore, necessary to provide a latch arrangement for preventing inadvertent change-over being effected or taking place without the knowledge of the user.
Latch mechanisms for achieving this are already known. In, for example, an electric drill having both a drill mode and a hammer drill mode, a control member is mounted on the exterior of the drill housing and communicates and cooperates with a pin member. This pin member has a main body portion which is generally cylindrical and, as mentioned hereinbefore, a longitudinally extending, eccentric projection portion. The pin member is located in a suitably dimensioned recess formed in the housing. In the external surface of the main body of the pin member, two diametrically opposed recesses are provided. In the wall of the recess in the housing two blind bores are formed, in each of which a spring-loaded ballbearing is fitted, the spring tending to urge its associated ballbearing into the appropriate recess in the pin member. Whilst the control member is being rotated the ballbearing is forced, by the cylindrical wall of the pin member, into the blind bore. However, when the control member is located in either of its two operative positions, the recess in the main body of the pin member is aligned with the blind bore. The ballbearing, under the action of the spring, is thus urged into the recess in the pin body. The ballbearing can, of course, be removed from the recess by further rotation of the control member.
A somewhat similar arrangement is present in the drill speed changeover mechanism. In this case, the control member includes a pin member which is eccentrically mounted with regard to the control member. The end of the pin remote from the control member is located between two gear wheels mounted on the main shaft of the drill. These two gear wheels are keyed onto the main shaft. Rotation of the control member through a desired angle causes movement of the gear wheels along the keyway so as to bring one or other of the gear wheels into engagement with a corresponding gear wheel mounted on a lay shaft.
The major disadvantage of known latching arrangements is that they tend to be situated in awkward locations. This is disadvantageous when machining the appropriate parts during manufacture of the drill. It also gives rise to the problem that, should a malfunction of the selector arrangement occur, the drill requires major dismantling before the fault can be rectified.